In electrophotography, an imaging system forms a latent image by exposing select portions of an electrostatically charged photoconductive surface to laser light. Essentially, the density of the electrostatic charge on the photoconductive surface is altered in areas exposed to a laser beam relative to those areas unexposed to the laser beam. The latent electrostatic image thus created is developed into a visible image by exposing the photoconductive surface to toner, which contains pigment components and thermoplastic components. When so exposed, the toner is attracted to the photoconductive surface in a manner that corresponds to the electrostatic density altered by the laser beam. The toner pattern is subsequently transferred from the photoconductive surface to the surface of a print substrate, such as paper, which has been given an electrostatic charge opposite that of the toner. The substrate then passes through a fuser that applies heat and pressure thereto. The applied heat causes constituents including the thermoplastic components of the toner to flow onto the surface and into the interstices between the fibers of the substrate. The applied pressure produces intimate contact between toner and fibers and promotes settling of the toner constituents into these interstitial spaces. As the toner subsequently cools, it solidifies adhering the image to the substrate.
The fuser typically includes cooperating fusing members that form a nip area capable of delivering heat and pressure to the substrate passing through the nip. Exemplary nip forming members include a fuser roll and a backup roll, a fuser roll and a backup belt and a fuser belt and backup roll. A heat source associated with one or both of the nip forming members raises the temperature of the fusing members at the nip area to a temperature required by a particular fusing application. As the substrate passes through the nip area, the toner is adhered to the substrate by the pressure between the nip forming members at the nip area and the heat resident in the fusing region.
Successful adherence of the toner to the substrate, known as fusegrade, is determined by fusing parameters including temperature, pressure and time in the nip area. Poor fusegrade, resulting in poor adhesion of the toner to the substrate, may be caused by insufficient temperature, pressure or time in the nip area. Moreover, excessive temperature, pressure or time in the nip area may cause damage to the toner image known as image mottle. Excessive temperature, pressure or time in the nip area may also cause the toner to stick to the fusing members rather than the substrate. For example, the toner may peel from the substrate and stick to the fuser members, a condition known as hot offset, or the toner with substrate attached may wrap about a fusing member.
In order to achieve proper fusegrade, the fuser parameters should ideally be maintained within an operating window defined between parameter values that result in poor fusegrade and parameter values that may result in image mottle, hot offset and/or wrap.